This invention relates to electrical inverters and more particularly to such inverters which convert a DC input to a multiple phase AC output.
Variable speed constant frequency (VSCF) electrical power supplies have been utilized where a constant frequency voltage is required to be derived from a prime mover which operates at a variable speed. For example, VSCF systems have been utilized in aircraft where 400 hertz power must be derived from a generator which is driven at a speed that is determined by the speed of an associated aircraft engine. One type of VSCF system employs a variable speed generator which delivers a DC voltage to a pair of DC link conductors. An inverter converts this DC voltage to a constant frequency multiple phase AC output.
Transistorized DC link inverters generally operate their power pole switching transistors near the limit of their current ratings. Consequently, these inverters require individual phase or pole current limiting to protect the switching transistors. When an overcurrent is sensed in a particular pole, action must be taken to limit the current. Two methods of current limiting are presently used. In the first method, the transistor switch in the pole with the excess current is turned off. This causes the current to flow through a commutating diode connected in parallel with one of the switching transistors and causes the pole voltage to reverse, thereby forcing the current to reduce at a rate controlled by an output filter inductor. The second method of current limiting senses the polarity of the excess current in each pole and reverses the pole which is subject to an excess current to force the current down. The current in the switched pole transfers to the opposite commutating diode exactly like the first method, since the opposite switching transistor will not conduct current in the reverse direction.
One method for sensing phase current is illustrated in U.S. Pat. No. 3,733,537, issued May 15, 1973 to Kernick et al. The current sensing circuit illustrated in that patent uses two current transformer coils in each output phase conductor. The current transformer coils are connected in a circuit known as a self-excited transductor. This circuit prevents loss of sensitivity in the presence of DC currents, which can develop during current limiting. The two described methods of current limiting both require six current transformers and both switch the pole transistors carrying the most current.